In a known exchange a two-wire subscriber line is coupled by line card circuitry to the PCM highway of the exchange.
The line card architecture for each line comprises a high voltage circuit, often formed by discrete high voltage components, coupled directly to the incoming subscriber line; a subscriber line interface circuit (SLIC) together with a trans-hybrid balance network and a COFIDEC circuit. A single line card controller couples the COFIDEC circuit of each line to the PCM highway.
The SLIC performs a conversion from the two-wire balanced subscriber line to a four-wire line having separate transmission lines for signals being transmitted to or received from the exchange, off hook detection and ring trip, D.C. line feed (either constant current or constant voltage) and two-wire impedance matching.
The trans-hybrid balance network ensures that the signals on the receive wire of the four-wire line do not appear on the transmit line.
The COFIDEC circuit performs analogue-to-digital and digital-to-analogue conversion of the four-wire signals, performs gain and frequency adjustment and data compression and expansion from typically 13 bits to the usual 8 bits of the exchange's PCM highway.
A known example of a SLIC is the Motorola MC3419 whilst a typical COFIDEC circuit is the MOTOROLA MC14400 family.
The above known subscriber line card architecture has several disadvantages. Firstly, the high voltage interface to the subscriber line has generally been formed by discrete components.
Of necessity the COFIDEC has been fabricated in CMOS integrated circuit technology since known digital bipolar processes would not be able to produce such a complex circuit of sufficient compactness. However, since the COFIDEC provides some analog signal processing, the effecting of such processing in CMOS circuits tends to worsen overall noise performance.
This invention seeks to provide a subscriber line card arrangement in which, at least in a preferred embodiment, hardware is reduced and one or more of the above mentioned disadvantages is mitigated.